Whirlwind, any rotating air mass, including the tornado and the large cyclonic and anticyclonic storm, or cyclone. In meteorology, the term whirlwind is more strictly applied to the smaller swirling atmospheric phenomenon commonly known as dust devil or dust whirl, which occurs mostly over deserts and semi-arid plains during hot, calm days.The principal cause of whirlwinds is intense insolation, or incoming solar radiation received by the Earth, which produces an overheated air mass just above the ground. This air mass rises, usually in the form of a cylindrical column, sucking up loose surface material, such as dust, sand, and leaves. Whirlwinds vary in height from 30 to 152 m (100 to 300 ft), but exceptionally vigorous dust devils may exceed 1,524 m (5,000 ft) in height. The vortices of whirlwinds range in size from a few metres to several hundred metres and, depending on their force and size, dust devils may disappear in seconds or last several hours. Brief whirlwinds are erratic in motion, but the longer-lasting ones move slowly with the prevailing winds.Water whirlwinds, commonly called waterspouts, are whirling columns of air and watery mist. The mist is mainly fresh water, formed by condensation in the atmosphere. Water whirlwinds are frequent occurrences over oceans and lakes but are seldom violent. Convective storms generate most waterspouts, and the rare tornadic spouts are generated in thunderstorms, in association with tropical cyclones or cold frontal squalls. Fire and smoke whirlwinds are caused by forest, oil, and incendiary-bomb fires, and they may have large, violent vortices.Dust and fire whirlwinds are reported in the Old Testament, notably in the Book of Job. Aristotle attempted to explain whirlwinds in his Meteorologica, a study of meteorological phenomena. Tornado WatchingA meteorologist tracks a tornado as part of continuing weather observations to learn more about the earth’s atmosphere. Since the 19th century, scientific forecasting has greatly improved. Weather radar can detect and track tornados, hurricanes, and other severe storms.Tornado (Latin, tonare, “to thunder”), in meteorology, violent whirling wind, characteristically accompanied by a funnel-shaped cloud extending down from a cumulonimbus cloud. Commonly known as a cyclone or twister, a tornado can be a few metres to about a kilometre wide where it touches the ground, with an average width of a few hundred metres. It can move over land for distances ranging from short hops to many kilometres, causing great damage wherever it descends. The funnel is made visible by the dust that is sucked up and by condensation of water droplets in the centre of the funnel. The same condensation process makes visible the generally weaker sea-going tornadoes, called waterspouts, that occur most frequently in tropical waters. Most tornadoes spin anticlockwise in the northern hemisphere and clockwise in the southern, but occasional tornadoes reverse this.The exact mechanisms that cause a tornado to form are still not fully understood, but the funnels are always associated with violent motions in the atmosphere, including strong updraughts and the passage of fronts. They develop within low-pressure areas of high winds; the speed of the funnel winds themselves is often placed at more than 480 km/hr (300 mph), although speeds of more than 800 km/hr (500 mph) have been estimated for extremely strong storms. Damage to property hit by a tornado results both from these winds and from the extremely reduced pressure in the centre of the funnel, which causes structures to explode when they are not sufficiently ventilated to adjust rapidly to the pressure difference. The pressure reduction is in keeping with Bernoulli_s principle, which states that pressure is reduced as velocity increases.Tornadoes are most common and strongest in temperate latitudes. In the United States they tend to form most frequently in the early spring; the “tornado season” shifts towards later months with increasing latitude. The number of funnels observed each year can vary greatly in any given region.
Hurricane, name applied to migratory tropical cyclones that originate over oceans in certain regions near the equator, and particularly to those arising in the West Indian region, including the Caribbean Sea and the Gulf of Mexico. Hurricane-type cyclones in the western Pacific are known as typhoons.Most hurricanes originate within the doldrums, a narrow equatorial belt characterized by intermittent calms, light variable breezes, and frequent squalls, and lying between the north-east and south-east trade winds. As the doldrums of the Atlantic are situated largely to the north of the equator, hurricanes do not occur in the South Atlantic Ocean. The Pacific doldrums extend north and south of the equator; thus hurricanes occur in the South and North Pacific oceans.Hurricanes consist of high-velocity winds blowing around a low-pressure centre, known as the eye of the storm. The low-pressure centre develops when the warm, saturated air prevalent in the doldrums is underrun and forced upwards by denser, cooler air. From the edge of the storm towards its centre, the atmospheric pressure drops sharply and the wind velocity rises.The winds attain maximum force close to the point of lowest pressure—about 724 torr (unit of pressure), or about 28.5 in. of mercury. The diameter of the area affected by winds of destructive force may exceed 240 km (150 mi). Gale winds prevail over a larger area, averaging 480 km (300 mi) in diameter. The strength of a hurricane is rated from 1 to 5. The mildest, Category 1, has winds of at least 120 km/hr (74 mph). The strongest (and rarest), Category 5, has winds that exceed 250 km/hr (155 mph). Within the eye of the storm, which averages 24 km (15 mi) in diameter, the winds stop and the clouds lift, but the seas remain very violent.Hurricanes generally move in a path resembling the curve of a parabola. In the northern hemisphere the storms usually travel first in a north-westerly direction and in the higher latitudes turn towards the north-east. In the southern hemisphere the usual path of the hurricane is initially to the south-west and subsequently to the south-east. Hurricanes travel at varying rates. In the lower latitudes the rate ranges from 8 to 32 km/hr (5 to 20 mph) and in the higher latitudes it may increase to as much as 80 km/hr (50 mph). Those areas in which the hurricane winds blow in the same direction as the general movement of the storm are subjected to the maximum destructive violence of the hurricane.Since 1943 military aircraft have been flying into hurricanes to measure wind velocities and directions, the location and size of the eye, the pressures within the storms, and their thermal structure. A coordinated system of tracking hurricanes was developed in the mid-1950s, and periodic improvements have been made over the years. Radar, sea-based recording devices, geosynchronous weather satellites, and other devices now supply data to hurricane monitoring centres, such as the National Hurricane Center in Florida, which follows each storm virtually from the beginning.Improved systems of prediction and communication have been able to help minimize loss of life in hurricanes, but property damage is still heavy, especially in coastal regions. The strongest hurricane to hit the western hemisphere in the 20th century, called Gilbert, devastated Jamaica and parts of Mexico in 1988 with winds that gusted up to 350 km/hr (218 mph). The United States is often affected by destructive hurricanes, the most recent being Andrew (1992), with an estimated $12 billion in damage, more than 50 dead, and thousands left homeless. Agnes (1972), with $3 billion in damage and 134 deaths and Hugo (1989), with more than $4 billion in damage and more than 50 deaths, also wreaked tremendous destruction. In Britain, one of the world_s windiest countries, but one not normally affected by hurricanes, the hurricane that swept the southern half of the country in 1987 exceeded 100 mph and felled millions of trees.